An input operation can be performed on a touch screen display device by touching certain points on the device with a stylus pen or a finger. The touch screen display device is provided with a plurality of touch sensing elements for detecting points touched by a user in addition to a plurality of pixels for displaying images.
A drive signal and a data signal are applied to each pixel, and each touch-sensing element senses a touch of the user to output a sensing signal according thereto. To this end, the touch screen display device is provided with a drive signal driving unit and a data signal driving unit for applying the drive signal and the data signal respectively, and a sensing line signal processing circuit for processing a signal outputted from each touch-sensing element to a sensing line.
FIG. 1 is a diagram illustrating a related art sensing line signal processing circuit of a touch screen. The sensing line signal processing circuit is provided with sensing read circuit units 10A to 10N, each of which has a sensor capacitor CS,1, a pre-charge switch PC, a readout switch RO, an integrator 11 (analog sensing channel), a switch unit 20, and an A/D converter 30.
The sensor capacitor CS,1 is one of the capacitors connected to a plurality of sensing lines arranged in one direction, e.g., vertical direction, on a touch screen panel. Cp,1 is a parasitic capacitor disposed between a pad adjacent to the sensor capacitor CS,1 and a ground terminal.
In a pre-charge mode, the pre-charge switch PC is turned on for a certain period of time so that a power supply voltage VDD is pre-charged to the sensor capacitor CS,1 through the pre-charge switch PC.
Thereafter, the readout switch RO is turned on for a certain period of time in a readout mode so that the voltage charged in the sensor capacitor CS,1 is transferred to the integrator 11 through the readout switch RO.
However, when the sensor capacitor CS,1 is touched by a user on the touch screen panel, a gap between electrode plates of the capacitor narrows, causing a variation in capacitance. Therefore, the voltage transferred from the sensor capacitor CS,1 to the integrator 11 decreases.
The integrator 11 integrates an input voltage and generates a touch sensing output voltage VOUT1 according thereto. Here, the touch sensing output voltage VOUT1 of the integrator 11 corresponds to a value obtained by dividing an amount of input charges by a capacitance value of a feedback capacitor CFB. That is, an integral value of a sensing line current is an amount of output charges.
According to the related art, as described above, the voltage of the sensor capacitor CS,1 connected to a single touch line is compared to a reference voltage Vref in an operational amplifier OP of the integrator 11 so as to determine the touch sensing output voltage VOUT1 according to a result of the comparison. That is, an absolute value comparison scheme is applied.
Through the above-mentioned processes, the touch sensing output voltage VOUT1 for a single touch line is generated in a single sensing read circuit unit, e.g., the sensing read circuit unit 10A, and touch sensing output voltages Vout2 to Voutn for the other touch lines are generated in the sensing read circuit units 10B to 10N in the same manner.
The switch unit 20 is provided with switches SW1 to SWn, the number of which corresponds to that of the sensing read circuit units 10A to 10N. By sequentially turning on the switches, the touch sensing output voltages Vout2 to Voutn outputted from the sensing read circuit units 10A to 10N are sequentially transferred to the A/D converter 30. The A/D converter 30 converts the inputted analog signals to digital signals, and outputs the digital signals.
A system control unit (not illustrated) recognizes a vertical coordinate of the touch point on the touch screen panel on the basis of the digital signals outputted from the A/D converter 30, and recognizes a horizontal coordinate on the basis of a signal detected through a drive line or an additionally arranged horizontal line to thereby determine the vertical and horizontal coordinates of the touch point.
For instance, in the case where 100 drive lines and 100 sensing lines are arranged on the touch screen panel, when a changed vertical coordinate signal is outputted from the sensing read circuit unit connected to a 30th sensing line and a 50th drive line signal is applied, the X and Y coordinates are respectively 50 and 30. That is, the intersecting point of the 50th drive line and the 30th sensing line is determined to be a touch region.
However, the circuit according to the related art is not stably operated if noise occurs when a touch operation is performed. FIG. 2 is a graph illustrating that the output voltage Vout is distorted when a large amount of noise is introduced at the time of touch input.
When a noise is introduced at the time of touch input, an output of the operational amplifier connected to the signal sensing line can be outside a normal operation range. Therefore, an output waveform is distorted, and it may be impossible to distinguish a touch.
In this case, in order to inhibit the distortion of the output of the operational amplifier, the capacitance value of the feedback capacitor CFB in the analog sensing channel may be increased or an analog filter may be used. However, these methods cause an increase in a circuit area and require additional capacitors, increasing cost and power consumption.